This invention relates to a novel thermoformable polyaryletherketone sheet and to thermoformed articles produced therefrom.
Shaped articles can be prepared from thermoplastic sheets using a thermoforming process. Thermoforming is defined in Tool and Manufacturing Engineers Handbook (Vol. 2, 4th Edition, Society of Manufacturing Engineers, Dearborn, Michigan, 1984, Charles Wick, Editor) as a process in which a thermoplastic sheet is heated to its processing temperature and, using mechanical methods or differential pressure created by vacuum and/or pressure, is forced to contact a mold surface and cooled while held to the contours of the mold until it retains the shape of the mold.
It is well known by those skilled in the art of thermoforming that processing temperatures at or above the crystalline melting points are required to form articles from semicrystalline polymers. Thus, as described in the art, the temperatures required for thermoforming polyaryletherketone sheets are in the range of 300.degree. to 400.degree. C., where these materials melt.
Polyaryletherketones consisting of condensation products of diphenyl ether and isophthalyl and terephthalyl chlorides, are disclosed in U.S. Pat. Nos. 3,516,966 (Berr), 3,666,612 (Angelo), and 3,637,592 (Berr). Films up to 300 micrometers in thickness have been prepared.
Thermoformable composites consisting of long fiber-reinforced polyaryletherketone matrices, are described in U.S. Pat. Nos. 3,434,914 (Sterman et al.), 4,624,886 (Cogswell et al.), 4,613,393 (Cattanach et al.) and 4,657,717 (Cattanach et al. The processing temperatures required to thermoform those composites were in each case at the crystalline melting point of the polymer matrix or higher.
It is known, for example, that an amorphous polyethylene terephthalate sheet can be readily thermoformed, and the thermoformed article can then be annealed to induce crystallization, which improves its mechanical properties, specifically tensile modulus, U.S. Pat. No. 4,457,797 (Hatchadoorian et al.). However, polyethylene terephthalate differs from polyaryletherketones in that the former can be extruded into sheets above its melting point and cooled to room temperature without inducing crystallization, while the latter tend to crystallize very fast on cooling and therefore cannot be readily extruded into amorphous sheets.
It would be highly desirable to be able to provide amorphous polyaryletherketone sheets, thermoformable at lower temperatures, comparable with those used for sheets made of other thermoformable materials, such as, e.g., polycarbonates or acrylics, say, in the vicinity of 160.degree. C. Such a development Would represent a significant improvement over the art because of lower energy requirements and lower capital investment.